You don't mention a lot of details, but in addition to all the other comments, I would add that you have to remember not all 600A breakers are the same.
The phase-phase voltage can be different, and more importantly, the interrupting rating. Depending on the service, it may have to be able to interrupt short circuit currents in the 10kA range, or as high as 500kA. Some old units used to have sand-fuses built into them. The breaker would open for modest faults, but if the fault current was *really* high, the breaker would just arc/weld until the sand fuses blew. They had a much higher interrupt rating.
Point is, once you get larger than the average residential service panel, there is more to a breaker than just the full-load current rating. Inverse time tripping curves, Very Inverse times, high interruption ratings, even whether it's meant for AC or DC.
daestrom